Falando estritamente filme canon ...
Em relação ao The Primes (por TF wiki):
Eons ago, the AllSpark created thirteen beings, the first Cybertronians, who had the ability to move between dimensions.
According to Jetfire, the Dynasty of Primes was formed by the first Thirteen Cybertronians. These beings could not transform, but they and their transforming descendants possessed powers beyond that of the average Cybertronian.
Mas nem todo Transformer é um descendente "genético" dos Primes. Os Primes eram apenas os treze originais criados.
They soon realized that the AllSpark's power, though vast, was not infinite, and required the energy of stars to be replenished. Once a nearby star went nova, the AllSpark created a worker race, the Transformers, who would assist the Dynasty in replenishing the AllSpark.
O Sentinel Prime não era um dos treze originais. Ele era um descendente direto.
Sentinel Prime was a direct descendant of the first Cybertronian, Primus, and was believed to be the last of the Primes.
Na continuidade do filme, o Optimus Prime foi originalmente chamado de "Optimus". Ele e o Megatron trabalharam com o Sentinel Prime.
[Optimus] was the humble leader of Cybertron's science division, which studied the AllSpark, and were protected by Lord High Protector Megatron's military.
[Megatron] was once brothers-in-arms with Optimus and a student of Sentinel Prime, appointed to be Cybertron's Protector and commander of its Defense Force.
Mas havia mais no Optimus do que aparenta. O Optimus é na verdade um descendente direto de um dos Primes (Prima, para ser específico).
It was Optimus Prime's ancestor who finally vanquished the Fallen to another dimension in the battle for the Matrix of Leadership, but his orphaned descendant who was hidden away from the Fallen grew up with no knowledge of what he was.
Sentinel Prime disse à Optimus sobre suas teorias sobre a linhagem de Prime
Though Optimus was too humble to accept, not believing Sentinel's claims about his heritage, Megatron overheard their conversation and became jealous of his "brother." Sentinel formulated another plan for Cybertron's future: it would be governed by Optimus's Science Division and Megatron's Defense Force. By having Optimus investigate the remains of the Primes' civilization, it would encourage him to believe in his lineage.
Eventualmente, a Optimus encontra provas de que ele é descendente de um Prime. É então que os outros Autobots começam a tratá-lo como tal, e ele se torna conhecido como "Optimus Prime".
Back at Burthov, he came upon Ratchet, Arcee, and Elita-One scanning the pieces of the other artifacts. Arcee then discovered that the pieces all contained the same symbol on them, and that the same symbol was on Optimus' head. Ratchet declared that the symbol was proof of the Dynasty of Primes, and it indicated Optimus himself was a Prime. Ratchet and the other Autobots began to dub him "Optimus Prime" [snip] Optimus also informed Sentinel of the marking on his head that proved he was right all along: he was a Prime too.
Megatron não era descendente direto de um Prime. Ele estava com ciúmes da Optimus.
But Megatron resented his comrade, knowing he was a Prime and therefore Sentinel's favored son.
Ainda assim, a Optimus e a Megatron lideraram a Cybertron por algum tempo.
For two generations, Optimus and Megatron ruled Cybertron together. The two personalities created a balance; Prime was fair while Megatron was firm.
Em um dos quadrinhos, Barricade quebra a quarta parede e explica que os dois não são verdadeiros "irmãos", apesar de cada um em relação ao outro como um.
However, in the twelfth issue of Titan's Revenge of the Fallen comic, "Barricade" informed readers that Megatron and Optimus weren't related, the term "brother" was just because they'd known each other for so long.
Colocando tudo isso junto, nós pintamos a imagem que a Optimus e a Megatron já foram aliados próximos. Eles trabalharam juntos sob o Sentinel para trazer paz a Cybertron. Enquanto Optimus era descendente de um Prime, Megatron não era. Ainda assim, a partir de anos de amizade e de aliados, eles se consideravam "irmãos" apesar de não serem "geneticamente" relacionados.